Correct selection of the circuit breaker

Often, during repair or new construction, it is time to replace the electrical wiring and the circuit breaker must be replaced. When we go to the electric shop, we almost always see that 99% of the range of circuit breakers is made up of devices with a C cutoff curve. Accordingly, it may appear that this is the only possible and correct option. This article is devoted to a review of one of the important characteristics of a circuit breaker - a trip curve.

The breaking curves of the circuit breakers consist of two parts:

• Tripping of the overload protection (thermal release): the higher the current, the shorter the tripping time;
• Tripping of short-circuit protection (electromagnetic trip): if the current exceeds the setpoint of this protection, the tripping time is less than 10 milliseconds.

For short-circuit currents exceeding 20 times the rated current, the current characteristics (tripping curves) do not provide sufficient display accuracy.

Disconnection of high short-circuit currents is characterized by current-limiting curves (shock current and energy). The total tripping time can be taken to be approximately equal to the 5-fold value of the ratio (I2t) / (I) 2.

Checking the selectivity between two circuit breakers

By superimposing the curve of the circuit breaker on the curve of the higher circuit breaker, you can check whether this combination of devices is selective in case of overload (selectivity for all current values, up to the electromagnetic setpoint of the upstream switch). This check is useful when one of the two circuit breakers has an adjustable set point; For devices with constant settings this information is contained directly in the selectivity tables. To test the selectivity for a short circuit, it is necessary to compare the energy characteristics of the two devices.

Example: according to the trip curve in the figure, the iC60 Schneider Electric Acti 9 circuit breaker (curve C, rated current 20 A) will cut off the current of 100 A (that is, 5 times the rated current In) for:
• at least 2 seconds;
• Maximum of 7 seconds.

Current characteristics

The selection of the current characteristic time must be carried out in accordance with the type of the protected system, since the requirements for protecting the electrical system will always be different. In this regard, in the scale of the apparatus there are 6 different curves.

Curve B

Tripping: 3 to 5 In (rated currents) IEC 60898
It is intended primarily for the protection of generators and in which there are no peak current spikes

Curve C

Tripping: 5 to 10 In (rated currents) IEC 60898
It is intended for protection of circuits in cases of general application

The curve D and K

Tripping: 10 to 14 In (rated currents) IEC 60947.2
It is designed to protect circuits with high starting current, namely for the protection of transformers and motors.

Curve Z

Tripping: 2.4 to 3.6 In (rated currents) IEC 60898
Designed primarily for the protection of electronic equipment.

Curve MA

Tripping: up to 12 In (rated currents) IEC 60898
It is designed to protect only the starting devices, since there is no thermal disengagement device.

This article is provided by ACTI9.ru